This invention relates generally to integrated circuits and manufacturing methods and more particularly to structures and methods having improved planarity and alkali ion gettering properties.
As is known in the art, in forming small line width geometries in a semiconductor process using photolithography, it is necessary to provide a highly planar surface for various photolithographic masks used in such process. Further, in the fabrication of dynamic random access memories (DRAMs), a plurality of gate electrodes are formed adjacent one another with relative small separation, i.e., gaps, between each adjacent pair thereof. The plurality of gate electrodes provide a high aspect ratio topography.
Further, the surface region adjacent the high aspect ratio topography may be relative low, i.e., relatively flat. Thus, it is necessary to fill these gaps width a suitable material, preferably a material with a low dielectric constant to prevent coupling between the adjacent electrodes and provide a planar surface over both the high and low aspect ratio topographies for subsequent photolithography.
With one process, after the gate electrodes are formed, a dielectric layer of silicon nitride is chemically vapor deposited (CVD) over the surface. The CVD silicon nitride is a conformal deposition and therefore gaps remain between adjacent gate electrode structures. The gap width between gate electrode structures after the layer of silicon nitride is deposited is in the order of 1200 .ANG.. Next, a layer of borophosphorosilicate glass (BPSG) is chemically vapor deposited over the structure to fill in the gaps. The CVD BPSG is thick enough to not only fill the gaps but also extends over the tops of the CVD silicon nitride layer and over the filled gaps to a thickness in the order of 1000 .ANG..
As is also known in the art, contaminants, such as sodium ions, or other alkali ions may come into contact with the outer BPSG layer. However, the phosphorous in the BPSG layer acts as a gettering material to counteract the effect of the alkali ion contaminant. The structure is then heated to form a more planar surface. However, with aspect ratios of 3-5 or higher, the BPSG layer may not provide a surface with the requisite degree of planarity.